This work is aimed at determining the cellular events in bone remodeling and the fate of nuclei in osteoclasts, using alveolar bone and tooth migration as a model. Similar in vitro studies are also planned with either fetal rat long bones or isolated bone cells from rat calvaria. The techniques used are mainly dynamic bone histomorphometry on undecalcified thin sections with double fluorescent labels, electron microsocopy and autoradiography with Ca 45, 3H thymidine and 3H proline. The time course of early effects of parathyroid hormone and calcitonin on the osteoclast population and the number of nuclei they contain have been studied. The results show that significant change occurs within 1 hour after hormone injection. The decrease in the number of osteoclasts after calcitonin is associated with a marked decrease in the number of nuclei in these same cells. Parathormone has the opposite effects. The effect of diphosphonates on osteoclast turnover, and nuclear content is presently being studied. Once the effects of diphophonates are characterized, studies of the effects, of PTH and CT upon these parameters will be evaluated in diphosphonate treated bone. Fetal rat long bone in culture are being studied using morphometry techniques and autoradiography. They show that both bone resorption and bone formulation go on during the four days in culture. This model will be used to study hormonal effects on bone cells and the nuclei turnover in the osteoclasts. Finally, isolated bone cell macrophages and monocytes are studied in EM to determine their morphometrical characteristics as a prelude to studying their induction and differentiation.